Earth's atmosphere is a critical system for life on our planet. Together with the oceans, the atmosphere shapes Earth's climate and weather patterns and makes some regions more habitable than others. But Earth's climate is not static. How variable is it, and how quickly does it change? What physical factors control climate, and how do they interact with one another?

To see how and why climate fluctuates, we need to learn about the basic characteristics of the atmosphere and some physical concepts that help us understand weather and climate. This unit describes the structure of the atmosphere and examines some of its key functions, including screening out harmful solar radiation, warming Earth through the natural greenhouse effect, and cycling carbon. It then summarizes how physical processes shape the distributions of pressures and temperatures on Earth to create climate zones, weather patterns, and storms, creating conditions suitable for life around the planet.

The atmosphere is a complex system in which physical and chemical reactions are constantly taking place. Many atmospheric processes take place in a state of dynamic balance—for example, there is an average balance between the heat input to, and output from, the atmosphere. This condition is akin to a leaky bucket sitting under a faucet: when the tap is turned on and water flows into the bucket, the water level will rise toward a steady state where inflow from the tap equals outflow through the leaks. Once this condition is attained, the water level will remain steady even though water is constantly flowing in and out of the bucket.

Similarly, Earth's climate system maintains a dynamic balance between solar energy entering and radiant energy leaving the atmosphere. Levels of oxygen in the atmosphere are regulated by a dynamic balance in the natural carbon cycle between processes that emit oxygen through photosynthesis and others that consume oxygen, such as respiration. The strength of atmospheric circulation is also controlled by a dynamic balance. Some parts of the planet receive more energy from the sun than others, and this uneven heating creates wind motions that act to move heat from warm to cold regions. (The process by which differential heating triggers atmospheric motion is discussed below in Section 5, "Vertical Motion in the Atmosphere.")

Today human actions are altering key dynamic balances in the atmosphere. Most importantly, humans are increasing greenhouse gas levels in the troposphere, which raises Earth's surface temperature by increasing the amount of heat radiated from the atmosphere back to the ground. The broad impacts of global warming are discussed in Unit 12, "Earth's Changing Climate," but it should be noted here that climate change will alter factors that are key determinants of environmental conditions upon which ecosystems depend. As the following sections will show, changing global surface temperatures and precipitation patterns will have major impacts on Earth's climate and weather.